Fuel cell separator, process for producing the same and material therefor

ABSTRACT

This invention relates to a fuel cell separator of excellent electrical conductivity, gas impermeability and strength. The composition for a fuel cell separator of this invention comprises a resin binder with a viscosity of 0.01–0.5 Pa·s at 150° C. and a viscosity of 3 Pa·s or more at 25° C. and graphite particles at a weight ratio of 1:(5–15) and a curing accelerator. The resin binder is exemplified by an epoxy resin binder composed of an epoxy resin and a curing agent for the epoxy resin. The process for producing the fuel cell separator of this invention is practiced by kneading and molding the aforementioned composition for a fuel cell separator and curing above the curing temperature of the composition. The fuel cell separator of this invention to be obtained by the aforementioned process shows a bulk density of 1.90 g/cm 3  or more, a sheet resistance (areal pressure, 0.5 MPa) of 40 m Ωcm 2  or less, a gas permeability of 1×10 −14  cm 2  or less and a flexural strength of 30 MPa or more.

FIELD OF TECHNOLOGY

This invention relates to a fuel cell separator, a process for producingthe same and a composition useful for the production of a fuel cellseparator.

BACKGROUND TECHNOLOGY

Fuel cells to be mounted on automobiles are attracting public attention.Fuel cells of this type utilize chemical energy directly as electricalenergy without converting it to thermal energy and normally generateelectricity by the reaction of hydrogen with oxygen. Fuel cells areavailable in several types such as phosphoric acid fuel cell, solidelectrolyte fuel cell and solid polymer fuel cell (PEFC) and separatorsthat are electrically conductive molded articles are used in solidpolymer fuel cells and phosphoric acid fuel cells. The separatorconstitutes a unit cell together with electrodes and the like and, asthe unit cells are used in a layered arrangement, the separator isrequired to keep gases (hydrogen and oxygen) separated from each otheron the one hand and to be electrically conductive on the other. For thisreason, the separator must meet requirements of a high electricalconductivity of 10×10⁻² Ωcm or less, low gas permeability and goodresistance to oxidation, hydrolysis and hot water.

A graphitized carbonaceous material composed of a binder andcarbonaceous particles of plural particle sizes is proposed inJP1992-214072 A in order to obtain a carbonaceous material that isdense, mechanically strong, electrically conductive and suitable for afuel cell separator. This technique, however, requires graphitizationafter molding. A carbonaceous material formulated from a thermosettingresin, Ketjenblack and spherical graphite particles is proposed inJP1996-31231 A in order to obtain a carbonaceous material suitable for afuel cell separator that shows a void of 5% or less and, when molded,shows a ratio of the volume resistivity in the XY direction to that inthe Z direction of 2 or less. Moreover, in order to reduce the amount ofbinder and improve the electrical conductivity, a method is proposed inJP1999-195422 A for incorporating a small amount of binder in acarbonaceous material, molding the mixture under pressure andimpregnating the molded article with an impregnating agent. Still more,a fuel cell separator whose surface roughness is controlled within thespecified range to reduce contact resistance to the electrode part isproposed in JP1999-297338 A. The use of synthetic graphite together withnatural graphite is proposed in JP2000-40517 A to produce a fuel cellseparator with minimal anisotropy. The use of specified graphiteparticles is proposed in JP2000-21421 A to produce a fuel cell separatorthat is well-balanced in properties such as gas impermeability, thermalconductivity and electrical conductivity. There is, however, a strongdemand for fuel cell separators that show better properties and arebetter balanced.

DISCLOSURE OF THE INVENTION

An object of this invention is to provide a fuel cell separator which iseasy to produce by molding, highly impermeable to gases, mechanicallystrong, dense and electrically conductive, a process for producing thesame and a material therefor.

This invention relates to a composition for a fuel cell separator whichis formulated from graphite particles and a liquid or solid resin binderwith a viscosity of 0.01–0.5 Pa·s at 150° C. and a viscosity of 3 Pa·sor more at 25° C. at a weight ratio of (5–15):1. An example of the resinbinder is an epoxy resin binder composed of an epoxy resin and a curingagent for the epoxy resin. In this case, a curing accelerator ispreferably used together with the epoxy resin binder.

The epoxy resin to be used as epoxy resin binder is exemplified by anepoxy resin represented by the following general formula (1),

wherein G is glycidyl or methylglycidyl group, R is a hydrogen atom, ahalogen atom or a hydrocarbon group containing 1–6 carbon atoms, eitheridentical with or different from one another, and n is a number of 0–15.

An example of graphite particles is a mixture of synthetic graphiteparticles with an average diameter of 50–300 μm and Kish graphiteparticles with an average diameter of less than 50 μm at a weight ratioof 40:60 to 90:10.

Moreover, this invention relates to a process for producing a fuel cellseparator which comprises kneading the aforementioned composition for afuel cell separator into a homogeneous composition, grinding thehomogeneous composition, molding the ground composition and curing themolded composition.

Still more, this invention relates to a fuel cell separator which isproduced by the aforementioned process and shows a bulk density of 1.90g/cm³ or more and a sheet resistance (areal pressure, 0.5 MPa) includingsheets of carbon paper of 40 m Ω cm² or less. It is preferable that aseparator for fuel cells shows a flexural strength of 30 MPa or more anda gas permeability of 1×10⁻¹⁴ cm² or less.

A fuel cell is constructed of plural layers of unit cells and aseparator is put between the adjacent unit cells to perform a functionof forming flow passages for fuel gas and oxidized gas between the unitcell and the electrode and separating fuel gas and oxidized gas withgrooves for passage of gases formed on the separator. The fuel cellseparator of this invention is produced by molding graphite particlesand a thermosetting resin into an article of the specified shape andcuring the molded article and the separator is used in fuel cells as itis or, if necessary, after additional processing to provide grooves orholes. It is to be understood that a fuel cell separator as referred toin this invention includes the components of a fuel cell separator priorto processing.

The composition for a fuel cell separator of this invention comprisesgraphite particles, a resin binder and a curing accelerator as essentialingredients. The ratio of the graphite particles to the resin binder is1 part by weight of the epoxy resin binder to 5–15 parts by weight ofthe graphite particles.

There is no restriction other than high electrical conductivity on thegraphite particles to be used in this invention and, for example, atleast one kind of the following variety of graphites is used;graphitized carbonaceous materials such as mesocarbon microbeads,graphitized coal- or petroleum-derived cokes, particles obtained byprocessing graphite electrodes and special carbonaceous materials,natural graphite, Kish graphite and expanded graphite.

The graphite particles to be used in this invention are desirably amixture of particles showing at least two kinds of particledistributions, that is, large particles with an average diameter of50–300 μm, preferably 70–150 μm, and small particles with an averagediameter of less than 50 μm, preferably 5–20 μm. The weight ratio oflarge particles to small particles is 40:60 to 90:10, preferably 70:30to 80:20. The use of two kinds of graphite particles is expected tooffer the following advantages. Large particles, when kneaded andground, generate new coke surfaces which come into contact to form apath for conducting electricity and, because large particles have asmall surface area, they can be kneaded even with a small amount ofresin. Small particles facilitate mutual contact of graphite particles,raise the strength of molded articles and are effective for raising thebulk density.

It is also desirable that the graphite particles to be used in thisinvention are a mixture of isotropic graphite particles and anisotropicgraphite particles and the weight ratio of the isotropic particles tothe anisotropic particles is 40:60 to 90:10, preferably 70:30 to 80:20.It is also desirable that the graphite particles are a mixture ofsynthetic graphite particles and Kish graphite particles and the weightratio of the two is 40:60 to 90:10, preferably 70:30 to 80:20. Moreover,a combination of isotropic graphite particles and Kish graphiteparticles yields good results. The use of two kinds or more of graphiteparticles produces another effect of raising the bulk density. Isotropicgraphite particles are prepared by molding an article by a known methodsuch as CIP molding and HIP molding followed by graphitizing andgrinding of the article. Synthetic graphite particles are preferred asisotropic graphite particles while Kish graphite particles are preferredas anisotropic graphite particles.

In the cases where a mixture of the aforementioned graphite particles isused, it is preferable that synthetic graphite particles or isotropicgraphite particles are used for large particles and Kish graphiteparticles or aniosotropic graphite particles are used for smallparticles. The use of graphite particles differing from each other inproperties and average diameter at the aforementioned ratio raises thebulk density and improves electrical conductivity, gas impermeabilityand strength. Moreover, synthetic graphite particles are somewhatinferior to natural graphite particles in respect to electricalconductivity but less anisotropic than natural graphite particles.

The resin binder to be used in this invention binds graphite particlestogether to a specified strength and hardens them and it is a solid or aliquid with a viscosity of 0.01–0.5 Pa·s at 150° C. and a viscosity of 3Pa·s or more at 25° C. A thermoplastic resin such as crystalline polymermay be used as resin binder, but a thermosetting resin is preferred. Avariety of thermosetting resins such as phenolic resins and epoxy resinsmay be used, but epoxy resins are preferred. Some of thermosettingresins require a curing agent and in such a case, the resin bindercomprises a thermosetting resin and a curing agent as essentialingredients. An example of a preferable resin binder is an epoxy resinbinder comprising an epoxy resin and a curing agent for the epoxy resin.

In the case of an epoxy resin binder, it is preferable that at least apart, preferably 50 wt % or more, more preferably 80 wt % or more, ofthe epoxy resin to be incorporated in the binder is represented by theaforementioned general formula (1). In formula (1), G is glycidyl ormethylglycidyl group, preferably glycidyl group, R is a hydrogen atom, ahalogen atom or a hydrocarbon group containing 1–6 carbon atoms, eitheridentical with or different from one another, preferably a hydrogen atomor an alkyl group containing 1–3 carbon atoms, and one benzene ring has0–4 substituents other than hydrogen. The symbol n is a number of 0–15and preferably n is in the range of 0–2 as average number of repeatingunits. The epoxy resin is preferably either a liquid at 25° C. or asolid with a softening point or a melting point of 100° C. or below,preferably 40–90° C.

The epoxy resin to be represented by general formula (1) is preferably abisphenol F type epoxy resin, that is, an epoxy resin represented bygeneral formula (1) wherein all R's are H or an alkylbisphenol F typeepoxy resin represented by the following formula (2).

The epoxy resin represented by the aforementioned general formula (1)can be obtained by the reaction of phenol or a substituted phenol withformalin to give bisphenol F or substituted bisphenol F followed byepoxidation with epichlorohydrin. Substituted phenols include cresol,xylenol, trimethylphenol, tetramethylphenol and chlorophenol.

Curing agents known for epoxy resins are useful here and they includephenols, amines and carboxylic acid derivatives such as phthalic acidand its derivatives, but polyhydric phenols are preferable andnovolak-based curing agents obtained from phenol, alkylphenol andformalin are more preferable. As for the novolak-based curing agents,those which soften above normal temperature are advantageous.

The resin binder is easy to handle if it is a liquid or a solid with aviscosity of 0.01–0.5 Pa·s, preferably 0.03–0.4 Pa·s, at 150° C. and aviscosity of 3 Pa·s or more at 25° C. The reason for controlling theviscosity at the aforementioned value at 150° C. is that a small amountof binder can yield a molded article, namely a separator, of highstrength, high bulk density and low resistivity. Control of theviscosity like this can be exercised easily by proper selection of thesoftening point and viscosity of the resin or the resin and curing agentto be used.

The equivalent ratio of epoxy resin to curing agent is not limited toany specific value in the case of an epoxy resin binder, but it ispreferably in the range of 0.5–1.5. In this case, a curing acceleratorfor the epoxy resin is preferably used and any of known acceleratorssuch as amines, imidazoles, phosphines and Lewis acids can be used. Theamount of curing accelerator is not specific, but it is preferably inthe range of 0.01–10 parts by weight per 100 parts by weight of theepoxy resin binder.

As for the proportion of graphite particles and resin binder, too muchor too little of the resin binder increases the resistivity and thegraphite particles are preferably used in an amount 5–15 times,preferably 8–12 times, the amount of the resin binder. If the amount ofthe graphite particles is more than 15 times, the density does not go upsufficiently and the gas impermeability goes down. If the amount is lessthan 5 times, the electrical conductivity decreases and the sheetresistance does not go down sufficiently. Generally, the use of too muchthermosetting resin hinders mutual contact of the graphite particles andthe electrical conductivity decreases while the use of too littlethermosetting resin does not yield a molded article of the specifiedstrength. In consideration of these circumstances, the proportion ofgraphite particles is controlled in the aforementioned range. The lowerlimit of the proportion of the resin binder is equal to the amountrequired to give a molded article of the specified strength and theresin binder is normally mixed with 5 times its weight or more of thegraphite particles, although the mix ratio varies with the kind ofresin.

In addition to the graphite particles and the resin binder, additivessuch as curing accelerators, parting agents and electrically conductivefillers may be incorporated in the composition for a fuel cell separatorof this invention within the range that does not ruin the effect of thisinvention. These additives are not calculated as resin binder orgraphite particles.

The graphite particles and the resin binder may be mixed at the sametime or two kinds of graphite differing in particle size distributionare mixed together in advance and then mixed with the resin binder, thelatter method being preferable.

In the production of a fuel cell separator from a composition therefor,an advantageous procedure is as follows; the resin, a mixture ofgraphite particles differing either in kind or in average particlediameter in an amount 5–15 times that of the resin, the resin binderand, optionally, a curing agent and other additives are kneaded to givea homogeneous composition, the composition is ground to an averageparticle diameter of 20–50 /im and the particles are molded and cured.

In the kneading step, any of general-purpose kneading machines such askneaders and rolls can be used, but the kneading operation is notlimited to the use of these machines. Kneading is carried out in such amanner as to give as homogeneous a composition as possible of the resinand graphite particles. It is allowable to apply heat or add alow-boiling solvent for the purpose of lowering the viscosity of theresin during kneading and care should be exercised not to completecuring in this case.

The composition obtained by kneading is then ground. The compositionthus obtained often becomes non-adhesive when cooled on account of arelatively low content of the resin and it can be ground by the use of aknown grinder. Examples of grinders to be used here are pulverizers forshear grinding and disc mills for compression grinding. It isadvantageous to carry out the grinding operation so that the averageparticle diameter becomes 50 μm or less, preferably 30 μm or less or20–50 μm. The electrical resistivity does not fall sufficiently when theparticle diameter is 50 μm or more while the cost of grinding increaseswhen the particle diameter is reduced too much; it is thereforedesirable to determine the particle diameter in consideration of thecapacity of a grinder and the cost of grinding. Of the graphiteparticles of different average diameter in the composition, largeparticles get ground preferentially in the grinding step therebygenerating a surface free of adhering resin and this is likely toproduce the effect of lowering the electrical resistivity. Therefore, itis advantageous to grind preferentially large particles with an averagediameter of 60–300 μm to particles with an average diameter of 50 μm orless while grinding as little as possible of small graphite particleswith an average diameter of less than 50 μm. If the grinding operationis carried out to an extent more than is necessary, the resin becomesinsufficient in amount to spread throughout the graphite particles andthere is the possibility that the strength of a molded articledeteriorates.

After grinding, the ground composition is molded by the use of a heatingtype molding machine equipped with a mold. Since the epoxy resin binderthat is a thermosetting resin is molded and cured at the same time, itis advisable to carry out the molding operation by keeping thetemperature at 100–135° C., preferably 150–200° C. The temperature iscontrolled at a level above the curing temperature and below thecarbonization temperature of the thermosetting resin in use. The moldingpressure set at a higher level would preferably lower the electricresistivity in planar direction and raise the bulk density; however, ashigher pressure would incur higher capital cost, the pressure issuitably controlled at 20–1,000 kg/cm², preferably 100–500 kg/cm².

If the ground composition is formed into an article of the shapeprescribed for a fuel cell separator and at the same time provided withthe prescribed grooves in the course of molding, the article can be usedfor a fuel cell separator as it is or after simple processing. Analternative procedure is to mold into a plate and process the plate foraddition of grooves and holes.

The fuel cell separator of this invention produced by the aforementionedprocess is dense and shows low gas permeability, high mechanicalstrength and good electrical conductivity.

The fuel cell separator produced by the process of this invention can beendowed with a bulk density of 1.90 g/cm³ or more, preferably 1.95 g/cm³or more, and excellent gas impermeability and mechanical strength. Whenthe bulk density is below 1.90 g/cm³, not only the gas impermeabilitybut also the mechanical strength deteriorate. The sheet resistance(areal pressure, 0.5 MPa) including sheets of carbon paper should be 40m Ω ² or less in order for the fuel cell to function properly as such,and this can be accomplished by the process of this invention. The sheetresistance can be reduced by using graphite of the kind showing highcrystallinity or reducing the amount of the thermosetting resin informulating the composition or it can be changed by the molding pressureand the like. The sheet resistance is determined by the procedure to bedescribed later in the examples.

The fuel cell separator of this invention preferably shows a flexuralstrength of 30 MPa or more and/or a gas permeability of 1×10⁻¹⁴ cm² orless.

When the flexural strength is 30 MPa or less, it is highly possible thatthe separator breaks down by vibration or impact. On the other hand,when the gas permeability exceeds 1×10⁻¹⁴ cm², hydrogen and oxygen fedseparately as fuel get mixed to deteriorate the power generatingefficiency.

The fuel cell separator of this invention is dense, mechanically strong,highly electrically conductive and least anisotropic and gas permeableand a fuel cell in which the separator is incorporated exhibits highefficiency and long life.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing to explain the method for measuring thesheet resistance.

FIG. 2 is a graph illustrating the relationship between the mix ratio ofgraphite particles and the tap bulk density (tap BD).

PREFERRED EMBODIMENTS OF THE INVENTION

The methods for measuring the sheet resistance and gas permeability ofthe molded article (separator) are as follows.

(Sheet resistance (m Ωcm²)) Referring to FIG. 1 schematically showingthe method for measuring the sheet resistance, two sheets of carbonpaper 2 are placed on the upper and lower sides of a specimen 1 (moldedarticle) with a thickness of 3 mm, two copper sheets 3 are placed on theupper and lower sides of the sheets of carbon paper and an arealpressure of 0.5 MPa is applied in the vertical direction. The voltagebetween the two sheets of carbon paper 2 is read off a voltmeter 4 andat the same time the current flowing between the two copper sheets isread off a galvanometer 5 and the resistance (average) is calculated.The carbon paper used here is TGP-H-M Series 090M (thickness, 0.28 mm)and 120M (thickness, 0.38 mm) available from Toray Industries, Inc.

(Bulk density) Measured in accordance with the Archimedes method.

(Flexural strength) Measured in accordance with JIS K6911.

(Gas permeability) The gas permeability K (cm²) was calculated inaccordance with the Darcy's law. In actual measurement, a sampleseparator, 32 mm in diameter and 2 mm in thickness, was placed in anairtight vessel and pressurized to 10 kg/cm² by nitrogen, the flow rateof nitrogen gas passing through the separator was measured and thepermeability was calculated as follows:K(cm ²)=Q·μ·1/ΔP·g _(c) ·A)wherein Q is the flow rate (cm³/sec), A is the area of the separator(cm²), 1 is the thickness (cm), μ is the viscosity of the gas(g/cm·sec), ΔP is the pressure difference (g/cm²) and g_(c) is thegravitational conversion factor.

The following materials are used in the examples.

Graphite particles with an average diameter of 110 ∥m: isotropicsynthetic graphite particles available from NSCC Techno-Carbon Co., Ltd.

Kish graphite particles with an average diameter of 10 μm: availablefrom Kowa Seiko Co., Ltd.

Ortho-cresol novolak epoxy resin: EOCN-1020 available from Nippon KayakuCo., Ltd., melting point 65° C., viscosity 0.3 Pa·s at 150° C.

Tetramethylbisphenol F type epoxy resin: YSLV-80XY available from NipponSteel Chemical Co., Ltd., softening point 75–80° C., viscosity 0.008Pa·s at 150° C.

Bisphenol F type epoxy resin: Epo Tohto YDF-170 available from TohtoKasei Co., Ltd., viscosity 3 Pa·s at 25° C.

Bisphenol A type epoxy resin: Epo Tohto YD-017 available from TohtoKasei Co., Ltd., softening point 117–127° C.

Liquid epoxy resin: Celloxide 2021A available from Daicel ChemicalIndustries, Ltd., melting point −20° C., viscosity 0.29 Pa·s at 25° C.

Liquid curing agent: Rikacid MH-700 available from Hitachi Chemical Co.,Ltd.

Phenol novolak resin: Tamanol 758 available from Arakawa ChemicalIndustries, Ltd., softening point 83° C., viscosity 0.22–0.35 Pa·s at150° C.

EXAMPLE 1

To 100 parts by weight of graphite particles prepared by mixing 50 partsby weight of the graphite particles with an average diameter of 110 μmand 50 parts by weight of the Kish graphite particles with an averagediameter of 10 μm was added a resin binder composed of an epoxy resinand a curing agent in the amount shown in Table 1. The resin binder hereis composed of 2 parts by weight of the ortho-cresol novolak epoxy resinas epoxy resin and 1 part by weight of the phenol novolak resin ascuring agent. The resin binder shows a viscosity of 0.38 Pa·s at 150° C.Moreover, triphenylphosphine was added as curing accelerator in anamount 0.01 times that of the resin binder.

The composition was kneaded in a roll heated at 100° C. The kneadedcomposition was ground in a disc mill to the secondary particle diameter(average diameter) shown in Table 1. The ground composition wasintroduced to a mold, molded at 175° C. and 350 kg/cm² for 20 minutes,removed from the mold and submitted to measurement.

The results of measurement of the secondary particle diameter, bulkdensity and sheet resistance are shown in Table 1.

TABLE 1 Graphite Secondary Bulk Sheet particles/binder particle densityresistance weight ratio diameter μm g/cm³ mΩcm² 5 30 1.96 45 6 30 1.9625 7 30 1.97 20

EXAMPLE 2

A composition was prepared as in Example 1 except using thetetramethylbisphenol F type epoxy resin and mixing the graphiteparticles and the resin binder at the ratio shown in Table 2. The resinbinder composed of the tetramethylbisphenol F type epoxy resin and thephenol novolak resin shows a viscosity of 0.036 Pa·s at 150° C. Thecomposition was kneaded, ground, molded as in Example 1 to give a moldedarticle.

The mix ratio of the graphite particles and the resin binder and theresults of measurement of the secondary particle diameter, bulk densityand sheet resistance are shown in Table 2.

TABLE 2 Graphite Median diameter Bulk Sheet particles/binder aftersecondary density resistance weight ratio grinding μm g/cm³ mΩcm²  8 291.96 16  9 30 1.98 15 10 38 1.95 12

EXAMPLE 3

To 100 parts by weight of graphite particles prepared by mixing 75 partsby weight of the graphite particles with an average diameter of 110 μmand 25 parts by weight of the Kish graphite particles with an averagediameter of 10 μm was added a resin binder composed of an epoxy resinand a curing agent in the amount shown in Table 3. The resin binder hereis composed of 2 parts by weight of the bisphenol F type epoxy resin and1 part by weight of the phenol novolak resin as curing agent. The resinbinder shows a viscosity of 0.022 Pa·s at 150° C. Moreover,triphenylphosphine was added as curing accelerator in an amount 0.01times that of the resin binder.

The composition was kneaded in a roll heated at 100° C. The kneadedcomposition was ground in a disc mill to the secondary particle diameter(average diameter) shown in Table 3. The ground composition wasintroduced to a mold, molded at 175° C. and 350 kg/cm² for 20 minutesand removed from the mold.

The mix ratio of the graphite particles and the resin binder and theresults of measurement of the properties are shown in Table 3.

TABLE 3 Graphite Median diameter Bulk Sheet particles/binder aftersecondary density resistance weight ratio grinding μm g/cm³ mΩcm² 12 331.94 10 13 34 1.93  9 15 24 1.92  8

COMPARATIVE EXAMPLE 1

A composition was prepared as in Example 1 except using a 16:1 mixtureby weight of the bisphenol A type epoxy resin and the phenol novolakresin as resin binder. The composition showed a too high viscosity andcould not be kneaded with the graphite particles in a roll at 100° C.When kneaded in a roll above 100° C., the epoxy resin and the curingagent cured independently and could not be kneaded homogeneously.

COMPARATIVE EXAMPLE 2

A 2:1 mixture of the liquid epoxy resin and the liquid curing agent wasused as resin binder. The binder showed a viscosity of 0.003 Pa·s at150° C.

The resin binder was kneaded with the graphite particles in a roll as inExample 3. Since the binder was too low in viscosity, the resin wasadsorbed in the gaps on the surface of the graphite particles and goodkneading was not possible.

In the experiments conducted as in Example 1, resin binders differing inthe combination of the epoxy resin and the curing agent were preparedand the relationship between the viscosity and the temperature wasexamined. A correlation was found between the viscosity at 150° C. andthat at 25° C. (in the state of liquid) and a 10-fold to 20-fold rise inviscosity was often observed.

In each of the Examples, the gas permeability of the separator (moldedarticle) obtained was 1×10⁻¹⁴ cm² or less.

EXAMPLE 4

To 100 parts by weight of graphite particles prepared by mixing 50 partsby weight of the synthetic graphite particles with an average diameterof 110 μm and 50 parts by weight of the Kish graphite particles with anaverage diameter of 10 μm was added a resin binder composed of an epoxyresin and a curing agent in the amount shown in Table 4. In thisexample, 2 parts by weight of the tetramethylbisphenol F type epoxyresin as epoxy resin, 1 part by weight of the phenol novolak resin ascuring agent and 0.03 part by weight of triphenylphosphine as curingaccelerator were used.

The composition was kneaded in a roll heated at 100° C. The kneadedcomposition was ground finely in a grinder. The ground composition wasintroduced to a mold, molded at 175° C. and 350 kg/cm² for 20 minutesand removed from the mold. The properties of the molded articles areshown in Table 4.

TABLE 4 Graphite Bulk Sheet Flexural Gas particles/binder densityresistance strength permeability weight ratio g/cm³ mΩcm² MPa cm² 3 1.86100 65 5.76 × 10⁻¹⁸ 5 1.90  40 60 2.57 × 10⁻¹⁷ 9 1.98  15 44 7.76 ×10⁻¹⁷

EXAMPLE 5

To 100 parts by weight of graphite particles prepared by mixing 75 partsby weight of the graphite particles with an average diameter of 110 μmand 25 parts by weight of the Kish graphite particles with an averagediameter of 10 μm was added a resin binder composed of an epoxy resinand a curing agent in the amount shown in Table 5.

In this example, 2 parts by weight of the bisphenol F type epoxy resinas epoxy resin, 1 part by weight of the phenol novolak resin as curingagent and 0.03 part by weight of triphenylphosphine as curingaccelerator were used.

The composition was kneaded in a roll heated at 100° C. and the kneadedcomposition was ground finely in a grinder. The ground composition wasintroduced to a mold, molded at 175° C., and 350 kg/cm² for 20 minutesand removed from the mold. The properties of the molded articles areshown in Table 5.

TABLE 5 Graphite Bulk Sheet Flexural Gas particles/binder densityresistance strength permeability weight ratio g/cm³ mΩcm² MPa cm² 111.95 10 38 2.89 × 10⁻¹⁶ 15 1.92  8 30 5.40 × 10⁻¹⁴ 17 1.86 27 15 1.40 ×10⁻¹²

EXAMPLE 6

To an epoxy resin binder composed of an epoxy resin and a curing agentwas added a mixture of graphite particles prepared by mixing 50 parts byweight of the graphite particles with an average diameter of 110 μm and50 parts by weight of the Kish graphite particles with an averagediameter of 10 μm in the amount shown in Table 6.

In this example, 2 parts by weight of the tetramethylbisphenol F typeepoxy resin as epoxy resin, 1 part by weight of the phenol novolak resinas curing agent and 0.03 part by weight of triphenylphosphine as curingaccelerator were used.

The composition was kneaded in a roll heated at 100° C. and the kneadedcomposition was ground in a grinder. The ground composition wasintroduced to a mold, molded at 175° C. and 350 kg/cm² for 20 minutesand removed from the mold. The properties of the molded articles areshown in Table 6.

TABLE 6 Graphite Bulk Sheet Flexural Gas particles/binder densityresistance strength permeability weight ratio g/cm³ mΩcm MPa cm² 3 1.86100 65 5.76 × 10⁻¹⁸ 5 1.90  40 60 2.57 × 10⁻¹⁷ 9 1.98  15 44 7.76 ×10⁻¹⁷

EXAMPLE 7

To an epoxy resin binder composed of an epoxy resin and a curing agentwas added a mixture of graphite particles prepared by mixing 75 parts byweight of the graphite particles with an average diameter of 110 μm and25 parts by weight of the Kish graphite particles with an averagediameter of 10 μm in the amount shown in Table 7.

The composition was formulated as in Example 6 except using thebisphenol F type epoxy resin as epoxy resin and the composition waskneaded, ground, and molded as in Example 6. The results of measurementof the properties of the molded articles are shown in Table 7.

TABLE 7 Graphite Bulk Sheet Flexural Gas particles/binder densityresistance strength permeability weight ratio g/cm³ mΩcm MPa cm² 11 1.9510 38 2.89 × 10⁻¹⁶ 15 1.92  8 30 1.18 × 10⁻¹⁵ 17 1.86 27 15 5.76 × 10⁻¹²

EXAMPLE 8

The experiment was carried out as in Example 6 while changing only themix ratio of the graphite particles with an average diameter of 110 μmand the Kish graphite particles (recrystallized graphite particles) withan average diameter of 10 μm and the relationship between the mix ratioof graphite particles and the tap bulk density was investigated. Theresults are shown in FIG. 2. The tap bulk density is determined byplacing 150 g of the sample in a 250-ml measuring cylinder, tapping thecylinder 900 times at the height of 30 mm and calculating the bulkdensity from the volume after the tapping.

EXAMPLE 9

The experiment was carried out as in Example 6 while changing only themix ratio of the graphite particles (isotropic synthetic graphiteparticles) with an average diameter of 110 μm and the Kish graphiteparticles (recrystallized graphite particles) with an average diameterof 10 μm. The ratio by weight of the graphite particles to the resinbinder is 9 in all experiments. The results are shown in Table 8.

TABLE 8 Isotropic synthetic graphite particles: Bulk Sheet Flexural Gasrecrystallized density resistance strength permeability graphiteparticles g/cm³ mΩcm MPa cm² 100:0  1.89 23 28 4.14 × 10⁻¹⁴ 75:25 2.0011 45 1.05 × 10⁻¹⁶ 50:50 1.98 15 44 7.76 × 10⁻¹⁷ 40:60 1.94 19 39 6.37 ×10⁻¹⁶ 25:75 1.87 38 14 5.33 × 10⁻¹³

INDUSTRIAL APPLICABILITY

Electrically conductive molded resin articles with low electricresistivity can be obtained without a heat treatment such as calcinationaccording to this invention and the process is effective for reducingthe production cost. Moreover, the molded articles show excellentelectrical conductivity, gas impermeability and strength and arevaluable for fuel cell separators.

1. A composition for a fuel cell separator which is formulated fromgraphite particles and a liquid or solid resin binder with a viscosityof 0.01–0.5 Pa·s at 150° C. and a viscosity of 3 Pa·s or more at 25° C.at a weight ratio of (5–15):1, wherein the graphite particles comprisesynthetic graphite particles with an average particle diameter of 50–300μm and Kish graphite particles with an average particle diameter of lessthan 50 μm at a mix ratio by weight in the range from 40:60 to 90:10. 2.A composition for a fuel cell separator as described in claim 1 whereinthe resin binder is an epoxy resin binder comprising an epoxy resin anda curing agent for the epoxy resin.
 3. A composition for a fuel cellseparator as described in claim 2 wherein the epoxy resin is representedby the following general formula (1)

(wherein G is glycidyl or methyiglycidyl group, R is a hydrogen atom, ahalogen atom or a hydrocarbon group containing 1–6 carbon atoms, eitheridentical with or different from one another, and n is a number of0–15).
 4. A composition for a fuel cell separator as described in claim3 wherein the epoxy resin represented by general formula (1) is abisphenol F type epoxy resin, that is, the compound represented bygeneral formula (1) wherein all R's are H and is either a liquid at 25°C. or a solid with a softening point or melting point of 30–100° C.
 5. Acomposition for a fuel cell separator as described in claim 2 whereinthe epoxy resin is represented by the following formula (2)


6. A fuel cell separator comprising the composition as desciibed inclaim 1 and showing a bulk density of 1.90 g/cm³ or more and a sheetresistance including sheets of carbon paper (areal pressure, 0.5 MPa) of40 m Ωcm² or less, wherein the composition is kneaded to homogeneity,ground, molded and cured.
 7. A fuel cell separator as described in claim6 wherein the flexural strength is 30 MPa or more and the gaspermeability is 1×10⁻¹⁴ cm² or less.
 8. A composition for a fuel cellseparator which is formulated from graphite particles and a liquid orsolid resin binder with a viscosity of 0.01–0.5 Pa·s at 150° C. and aviscosity of 3 Pa·s or more at 25° C. at a weight ratio of (5–15): 1,wherein the graphite particles comprise isotropic graphite particleswith an average particle diameter of 70–150 μm and anisotropic graphiteparticles with an average particle diameter of 5–20 μm at a mix ratio byweight in the range from 70:30 to 80:20.
 9. A composition for a fuelcell separator as described in claim 8 wherein the resin binder is anepoxy resin binder comprising an epoxy resin and a curing agent for theepoxy resin.
 10. A composition for a fuel cell separator as described inclaim 9 wherein the epoxy resin is represented by the following generalformula (1)

(wherein G is glycidyl or methyiglycidyl group, R is a hydrogen atom, ahalogen atom or a hydrocarbon group containing 1–6 carbon atoms, eitheridentical with or different from one another, and n is a number of0–15).
 11. A composition for a fuel cell separator as described in claim10 wherein the epoxy resin represented by general formula (1) is abisphenol F type epoxy resin, that is, the compound represented bygeneral formula (1) wherein all R's are H and is either a liquid at 25°C. or a solid with a softening point or melting point of 30–100° C. 12.A composition for a fuel cell separator as described in claim 9 whereinthe epoxy resin is represented by the following formula (2)


13. A fuel cell separator comprising the composition as described inclaim 8 and showing a bulk density of 1.90 g/cm³ or more and a sheetresistance including sheets of carbon paper (areal pressure, 0.5 MPa) of40 m Ωcm² or less, wherein the composition is kneaded to homogeneity,ground, molded and cured.
 14. A fuel cell separator as described inclaim 13 wherein the flexural strength is 30 MPa or more and the gaspermeability is 1×10⁻¹⁴cm² or less.
 15. A process for producing a fuelcell separator which comprises kneading a composition formulated fromgraphite particles and a liquid or solid resin binder with a viscosityof 0.01–0.5 Pa·s at 150° C. and a viscosity of 3 Pa·s or more at 25° C.at a weight ratio of (5–15):1 into a homogeneous composition, grindingthe homogeneous composition to an average particle diameter of less than50 μm, molding the ground composition, and curing the moldedcomposition, wherein the graphite particles comprises synthetic graphiteparticles with an average particle diameter of 50–300 μm and Kishgraphite particles with an average particle diameter of less than 50 μmat a mix ratio by weight in the range from 40:60 to 90:10, and whereinthe resin binder is an epoxy resin binder comprising an epoxy resin anda curing agent for the epoxy resin.